I Determining a stationary point of reference to base all absolute motion

[tsslieberman]

There is no such thing to have an idea of.

I'm not really sure this even makes sense. If you are referring to all the matter in the universe, yes you can at least conceive of it moving or rotating. Bulk motion of matter is unmeasurable, but rotation would be detectable.

No, just look at clocks and compare them.

Nothing can be dependent on "absolute time" because there is no such thing.

Any time. The whole point about $c$ is that it's invariant - as long as you use clocks and rulers that are at rest with respect to each other you will always get the same value of the speed of light (even in non-SI unit systems where measuring the speed of light is non-tautological).

A question that does make sense and may be related to what you are trying to ask is what is our speed relative to the coordinates used to state that the universe is 14bn years old. That's measurable (about 600kps, from memory). Does it affect our estimate of the age of the universe? Technically it means we've experienced less time since the beginning of the universe than an observer who sees an isotropic CMB, but the effect is several orders of magnitude less than our experimental uncertainty in the age, so it's not relevant in practice.

And the fact that we have no idea of our absolute movement doesn’t bother you?

 

[tsslieberman]

Relative to what? Such a statement is meaningless unless you specify what the speed is relative to.

I think what you mean is, relative to the sun and earth (assuming both to be at rest relative to each other, which is not actually true, but is a reasonable approximation for a simple idealized scenario). But you should not leave us to guess. You should explicitly tell us.


How? You're not on the sun or the earth. You can't just magically know times of things that happen at places where you aren't.

One of the key implications of the fact that light travels at a finite speed (or, if we use the more basic terminology of my earlier post, of the light cone structure of the geometry of spacetime that constrains causal propagation) is that you have to explicitly think through how you actually get information about events that happen at other locations. You can't just assume that it somehow happens and ignore the details, as you can in non-relativistic physics.

Oh, my bad. I thought we could think of “thought experiments” in this forum. I didn’t realize we had to have experimental evidence.

 

[tsslieberman]

Among other things, the distance from the Sun to the Earth is not invariant. Again, this is something you are assuming from classical concepts of space and distance. In relativity, the distance from the Sun to the Earth depends on whether you are at rest relative to those objects or moving relative to those objects.

It's quite hopeless to talk about relativity, while still hanging on to classical physics concepts.

At rest? That’s exactly what I’m proposing

 

[tsslieberman]

I'm not sure what you mean. @Ibix was making a simple factual statement. There is no way to "change" it.

Uh, I disagree. There is a way to change one’s perception of “reality”

 

[renormalize]

Uh, I disagree. There is a way to change one’s perception of “reality”

But changing one's perception affects only the perceiver, not the external reality.

 

[PeterDonis]

Maybe, instead of sending me references, you could guide me? Let’s not regurgitate information we already have and instead, maybe come up with new ideas

Again, I don't understand what you mean. There are no new ideas needed here. You simply need to accept what you have already been told in this thread, that there is no such thing as absolute motion. Period. End of story.

Uh, I disagree. There is a way to change one’s perception of “reality”

@Ibix was not talking about perception. The statement "there is no such thing as absolute motion" is not a statement about perception. It is a statement about reality.

 

[PeterDonis]

I thought we could think of “thought experiments” in this forum. I didn’t realize we had to have experimental evidence.

I wasn't asking for experimental evidence of anything. I was telling you that the way you are imagining your thought experiment is incomplete; you are leaving out critical things that, in relativity, you cannot leave out. Without them you are simply doing it wrong.

 

[Nugatory]

I see. So time does not progress slower with increasing velocity near C?

That is correct - it does not. A clock that is at rest relative to you will always tick at the rate of one second per second.

You may believe that you’ve read or heard something about time slowing down for moving objects. If so you have misunderstood what your source was saying.

 

[Ibix]

Oh, I thought that if I traveled at near light speed, time would progress faster.

You keep assuming an absolute sense of the word speed, which is incompatible with observation.

If I measure you to be travelling fast relative to me then I will say that your clocks are ticking slow. You will say the same about my clocks. This kind of behaviour is not explicable in terms of "time running slow", although many popsci sources (ab)use the notion.

Maybe, instead of sending me references, you could guide me?

You mean, write down everything I learned from a textbook? Wouldn't that just be another textbook?

Also, we've all pointed out that there is no absolute notion of speed and you continue to write as if there were. Guidance from us does not seem to be helping you.

maybe come up with new ideas

The problem is that the principle of relativity has a huge supporting evidence base. It can't simply be discarded.

And the fact that we have no idea of our absolute movement doesn’t bother you?

No. And even if it did, the fact that I'm currently sitting apparently still on an object turning at about 500mph (at my latitude) and orbiting at about 20km/s is inescapable.

I thought we could think of “thought experiments” in this forum.

You can do - but you keep leaving critical information out of your descriptions and ignoring us when we point this out.

 

[weirdoguy]

There is no absolute motion in NON-relativistic physics either. @tsslieberman you either accept the way the world is or live in frustration till the end. Coming up with "new ideas" won't change reality. Physics is an experimental science. And milions of people throughout the centuries devoted their lives to explore, experimentally, the way reality is. Dismissing their work because you don't like the way world works is very inappropriate, to say the least.

 

[PeroK]

Oh, my bad. I thought we could think of “thought experiments” in this forum. I didn’t realize we had to have experimental evidence.

Physics is an empirical science. A thought experiment might raise a question. But, the question is answered by a real experiment. If you are an engineer, this might interest you:

 

[Dale]

Let’s not regurgitate information we already have and instead, maybe come up with new ideas

Coming up with new ideas is the purview of the professional scientific literature. If you want to do that you will need to study carefully the existing literature (like the link above) so that you can recognize a new idea when you do see it. The ideas you are mentioning here are more than 300 years old

 

[Ibix]

The ideas you are mentioning here are more than 300 years old

Just to clarify for @tsslieberman, Dale means that ideas like an absolute sense of rest were abandoned 300 years ago because even then it was obvious that they didn't accurately describe reality.

It's also worth noting that serious scientists do continue to look for more subtle violations of Lorentz covariance (an absolute rest frame would be a fairly gross violation), since such things might point to future theories of gravity. Nothing has been detected.

 

[tsslieberman]

Just to clarify for @tsslieberman, Dale means that ideas like an absolute sense of rest were abandoned 300 years ago because even then it was obvious that they didn't accurately describe reality.

It's also worth noting that serious scientists do continue to look for more subtle violations of Lorentz covariance (an absolute rest frame would be a fairly gross violation), since such things might point to future theories of gravity. Nothing has been detected.

Thank you for the detailed explanation.

I really do appreciate people taking the time to respond.

So, let me ask, theoretically speaking, if we launched a spacecraft that had propulsion and kept moving that craft to a point where it’s relative time, as compared to a clock on earth, ran the fastest, would that not indicate that it would be a stationary point? Again, this is just a thought experiment since I don’t think it would be possible with technology as it exists today.

Would that not be helpful to know? It may not affect our reality, but a reference point to see if the observable universe is moving or spinning.

 

[Ibix]

if we launched a spacecraft that had propulsion and kept moving that craft to a point where it’s relative time, as compared to a clock on earth, ran the fastest, would that not indicate that it would be a stationary point?

No. Again, you are assuming that there's some meaning to the concept of absolute rest. This has been known to be wrong since the time of Newton. Simply repeating your mistake will not make it right, nor will it make any reasoning from your incorrect premise meaningful.

 

[Ibix]

Let me add: time dilation is derived from Einstein's two postulates. Using it in a thought experiment implies you accept the principle of relativity. Including an absolute sense of rest at the same time is a direct self-contradiction. You cannot get meaningful reasoning from it.

 

[Nugatory]

So, let me ask, theoretically speaking, if we launched a spacecraft that had propulsion and kept moving that craft to a point where it’s relative time, as compared to a clock on earth, ran the fastest, would that not indicate that it would be a stationary point?

That scenario is not stated unambiguously because:
1) You have not specified whether the spacecraft has stopped accelerating and is now coasting at a constant speed $v$ relative to the earth, or is continuing to accelerate while we perform the comparison with the clock on earth. The analysis will be different in the two cases (and much hairier in the continued acceleration case, because there is no inertial frame in which the ship is at rest - google for "Rindler coordinates" to get a sense of just how deep the mathermatical water is here). If the ship is coasting, every point is a stationary point when we do our calculations using the frame in which the ship is at rest - that's the definition of "coasting"
2) You have not specified how we are comparing the clock on earth with the clock on the ship. To repeat something I said almost at the very beginning of this thread: "The only time that we can directly measure is that measured by a clock that is where we are and at rest relative to us. Everything else is inferred or calculated".
So earth person has no problem reading the earth clock and spaceship person has no problem reading the spaceship clock, but there's no way of comparing the clocks withouut saying some thing along the lines of "at the same time that clock A read THIS, clock B read THAT", and you have not specified how that is to be inferred or calculated.

Here it is important to add that the obvious way of doing this comparison - look at the remote clock through a telescope, if we see it reading noon when our clock reads 1:00 PM and we're one light-hour apart we infer that the remote clock now reads 1:00 PM because the light took one hour to get to us - DOES NOT WORK if the two clocks are not at rest relative to one another.

a reference point to see if the observable universe is moving or spinning.

But we have no idea what point we're talking about, nor whether it even exists, or if it does whether we'll agree about where it is, because your thought experiment is missing the critical definitions.

 

[tsslieberman]

Physics is an empirical science. A thought experiment might raise a question. But, the question is answered by a real experiment. If you are an engineer, this might interest you:

It does. Great video. Thank you.
I still have my uncle’s slide rule :)

 

[PeterDonis]

a point where it’s relative time, as compared to a clock on earth, ran the fastest

There is no such point. Even leaving aside the issues involved with making the comparison at all (which you are ignoring, even though I've already pointed out that you can't), there is no "maximum" possible amount of difference in "relative time". It is always possible to find another point or state of motion which has more difference than whichever one you are currently considering.

this is just a thought experiment

Even for thought experiments, you have to take as your starting point the actual laws of physics in question, in this case relativity. You aren't doing that. You're making up scenarios based on false premises. That is not a good way to learn.

 

[jbriggs444]

So, let me ask, theoretically speaking, if we launched a spacecraft that had propulsion and kept moving that craft to a point where it’s relative time, as compared to a clock on earth, ran the fastest, would that not indicate that it would be a stationary point?

As I understand this experiment, we launch a space craft. Or a series of space craft, each with a different speed. We let each craft coast at its selected speed. They are all equipped with clocks.

From our vantage point at rest in a comfy chair at our home base we watch those clocks, paying close attention to the tick rates. Importantly, we correct our measurements for speed of light delays -- the Doppler effect.

We look for the craft whose [adjusted for Doppler] tick rate is the fastest of all of them. That one will identify our candidate "absolute frame of reference".

The theory of relativity predicts that the craft with the fastest clock will be one that is at rest with respect to us. That the "absolute frame" is our own inertial rest frame.

It predicts that the same will hold for anyone else who runs the same test. They will find that the "absolute frame" is their own inertial rest frame. If this seems strange, you will probably need to learn about the relativity of simultaneity before things make sense.

The point of a thought experiment is to apply the laws of a theory and see what that theory predicts. It is a way to explore the consequences of a theory. Special relativity predicts as above that there is no uniquely identifiable absolute inertial frame. Any inertial frame will do just as well as any other.

 

[PeterDonis]

The theory of relativity predicts that the craft with the fastest clock will be one that is at rest with respect to us.

If we only take into account special relativity, i.e., if we assume that spacetime is flat, then that will be true, yes. More precisely, it will be true relative to us. But some other observer moving relative to us will say that his clock runs fastest, and our clock runs slower. Such comparisons of clock rates are always relative. (More precisely, they are always relative if we restrict ourselves to objects that always move inertially, i.e., zero proper acceleration. If we allow nonzero proper acceleration, we can get clock comparisons that both observers will agree on.)

But if we take into account general relativity, i.e., gravity, i.e., if we allow spacetime to be curved, then we can no longer make the general statement you make. Clocks higher up in a gravity well than us might run faster even if they are moving with respect to us. And note that, when we take gravity wells into account, we can get clock comparisons that are invariant, i.e., both observers agree on which clock is running faster, even if we restrict to only inertial motion.

But there will never be a clock that runs "the fastest of all" in any absolute sense. Even taking into account gravity wells, no matter what clock we pick, we will always be able to find one that runs faster. (And the same is true for running slow--there is no clock that runs "the slowest of all" in any absolute sense either.) So the thing the OP is trying to postulate simply does not exist at all.

 

[tsslieberman]

As I understand this experiment, we launch a space craft. Or a series of space craft, each with a different speed. We let each craft coast at its selected speed. They are all equipped with clocks.

From our vantage point at rest in a comfy chair at our home base we watch those clocks, paying close attention to the tick rates. Importantly, we correct our measurements for speed of light delays -- the Doppler effect.

We look for the craft whose [adjusted for Doppler] tick rate is the fastest of all of them. That one will identify our candidate "absolute frame of reference".

The theory of relativity predicts that the craft with the fastest clock will be one that is at rest with respect to us. That the "absolute frame" is our own inertial rest frame.

It predicts that the same will hold for anyone else who runs the same test. They will find that the "absolute frame" is their own inertial rest frame. If this seems strange, you will probably need to learn about the relativity of simultaneity before things make sense.

The point of a thought experiment is to apply the laws of a theory and see what that theory predicts. It is a way to explore the consequences of a theory. Special relativity predicts as above that there is no uniquely identifiable absolute inertial frame. Any inertial frame will do just as well as any other.

Thank you. I appreciate the thoughtful response.

 

[tsslieberman]

Thank you. I appreciate the thoughtful response.

 

[Dale]

if we launched a spacecraft that had propulsion and kept moving that craft to a point where it’s relative time, as compared to a clock on earth, ran the fastest, would that not indicate that it would be a stationary point?

No, that would only indicate that it is at rest relative to earth.

 

[PeterDonis]

No, that would only indicate that it is at rest relative to earth.

With the caveats I gave in post #51.

 

[Dale]

Clocks higher up in a gravity well than us might run faster even if they are moving with respect to us.

With the caveats I gave in post #51.

I think your caveats are going too far. If a clock higher up but moving runs faster then a clock just as high and at rest will run even faster than that. The fastest relative to earth will still be a clock at rest wrt earth, even in GR.

A better caveat would be that in curved spacetime there is no unique way to compare velocities at separate events. But I don’t think that caveat is worthwhile here.

 

[PeterDonis]

I think your caveats are going too far.

I agree that, if we are talking about a single gravity well and we pick a given altitude, the fastest clock at that altitude, relative to a clock at rest on the (non-rotating) gravitating body, will be the one at rest relative to the gravitating body.

However, this sense of "fastest" is still relative, and the caveats I gave still apply: a clock at rest relative to the gravitating body at a given altitude will not be faster than all moving clocks, only moving clocks whose altitude is not high enough above the at rest clock to compensate for the slowdown due to their speed.

In any event, the OP is trying to use "fastest" in an absolute sense, and there is no "fastest" clock in that sense. Pick any altitude you like, and find the fastest clock at that altitude, i.e., the one at rest relative to the gravitating body. You can still find a clock at a higher altitude that is faster still. And there is no highest altitude; space goes out to infinity. So it is impossible to find an absolute fastest clock--no matter what clock you pick, you will be able to find another one that is faster.

 

[Herman Trivilino]

Oh, I thought that if I traveled at near light speed, time would progress faster.

But you are already traveling at near light speed relative to, say, a beam of protons in the LHC.

Speed is not a property of the object that's moving. Different frames of reference will measure different speeds for the same object.

There is no experiment you can do to distinguish between a state of rest and a state of uniform motion. That's an idea first articulated by Galileo. (He described it quite eloquently in layman's terms, discussing what it's like to be enclosed in the hull of a moving ship on a calm sea. I encourage you to do a google search, it's a great read.) It was Newton's First Law. It was Einstein's First Postulate. In the hierarchy of physical law it now forms the Principle of Relativity.